[PATCH 0/1] mm, compaction: correct the bounds of __fragmentation_index()

[robert . m . harris]

From: "Robert M. Harris" <robert.m.harris@xxxxxxxxxx>

__fragmentation_index() calculates a value used to determine whether
compaction should be favoured over page reclaim in the event of
allocation failure.  The function purports to return a value between 0
and 1000, representing units of 1/1000.  Barring the case of a
pathological shortfall of memory, the lower bound is instead 500.  This
is significant because it is the default value of
sysctl_extfrag_threshold, i.e. the value below which compaction should
be avoided in favour of page reclaim for costly pages.

Here's an illustration using a zone that I fragmented with selective
calls to __alloc_pages() and __free_pages --- the fragmentation for
order-1 could not be minimised further yet is reported as 0.5:

# head -1 /proc/buddyinfo
Node 0, zone      DMA   1983      0      0      0      0      0      0      0      0      0      0 
# head -1 /sys/kernel/debug/extfrag/extfrag_index
Node 0, zone      DMA -1.000 0.500 0.750 0.875 0.937 0.969 0.984 0.992 0.996 0.998 0.999 
# 

With extreme memory shortage the reported fragmentation index does go
lower.  In fact, it can go below zero:

# head -1 /proc/buddyinfo
Node 0, zone      DMA      1      0      0      0      0      0      0      0      0      0      0 
# head -1 /sys/kernel/debug/extfrag/extfrag_index
Node 0, zone      DMA -1.000 0.-500 0.-250 0.-125 0.-62 0.-31 0.-15 0.-07 0.-03 0.-01 0.000 
# 

This patch implements and documents a modified version of the original
expression that returns a value in the range 0 <= index < 1000.  It
amends the default value of sysctl_extfrag_threshold to preserve the
existing behaviour.  With this patch in place, the same two tests yield

# head -1 /proc/buddyinfo
Node 0, zone      DMA   1983      0      0      0      0      0      0      0      0      0      0 
# head -1 /sys/kernel/debug/extfrag/extfrag_index
Node 0, zone      DMA -1.000 0.000 0.500 0.750 0.875 0.937 0.969 0.984 0.992 0.996 0.998 
# 

and

# head -1 /proc/buddyinfo
Node 0, zone      DMA      1      0      0      0      0      0      0      0      0      0      0 
# head -1 /sys/kernel/debug/extfrag/extfrag_index
Node 0, zone      DMA -1.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 0.000 
# 

Robert M. Harris (1):
  mm, compaction: correct the bounds of __fragmentation_index()

 Documentation/sysctl/vm.txt |  2 +-
 mm/compaction.c             |  2 +-
 mm/vmstat.c                 | 47 +++++++++++++++++++++++++++++++++++----------
 3 files changed, 39 insertions(+), 12 deletions(-)

-- 
1.8.3.1